[5] Lachesis muta is the third longest venomous snake in the world, exceeded in length only by the king cobra and the black mamba.

Midbody there are 31-37 nonoblique rows of dorsal scales which are heavily keeled with bulbous tubercles and feebly imbricate.

[5] Some reports suggest that this species produces a large amount of venom that is weak compared to some other vipers.

This may explain the disparity described by Hardy and Haad (1998) between the low laboratory toxicity of the venom and the high mortality rate of bite victims.

The symptoms are quite similar to those caused by Bothrops, at the site of the bite there is pain, edema, ecchymosis, skin necrosis, abscesses, vesicles and blisters.

The main complications at the bite site include necrosis, compartment syndrome, secondary infections and functional deficit.

The systemic effects are characterized by hypotension, dizziness, visual disturbances, bradycardia, abdominal pain, nausea, vomiting and diarrhea.

[12] In ilhéus, Bahia, Brazil, a 7-year-old boy was bitten when he left the house and stepped on one of these specimens, which then readily bit him; death was reported to have occurred approximately 15 minutes later.

[13][14] Lachesis is one of the three Fates in Greek mythology and was supposed to assign to man his term of life—something this species is certainly capable of doing.

The species is similar in appearance to rattlesnakes and vibrates its tail vigorously when alarmed, but has no rattle and was therefore called mutus (later muta), which is Latin for "dumb" or "mute".

[20] L. muta is found in South America in the equatorial forests east of the Andes, and the island of Trinidad.